net/mlx5: fix RSS key configuration

net/mlx5: fix RSS key configuration

The trigger for PMD to use the application RSS configuration should be
based on the validity of rss_key and not the rss_key_len. Otherwise
segmentation fault can occur if application provide valid RSS key length
but without any RSS key.

Fixes: 29c1d8bb3e79 ("net/mlx5: handle a single RSS hash key for all protocols")
Cc: stable@dpdk.org
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>